1. Field of the Invention
The current invention relates to methods of making foraminous, or porous, structures, and in particular to methods of making a multilayer porous films for use in photonic applications.
2. Background Information
TiO2 nanostructures are of great interest due to their versatile applications in dye-sensitized solar cells, photocatalysis, electrochromic effects and self-cleaning ability. To date, TiO2 nanostructures have been fabricated by various methods such as sputtering, evaporation deposition, electrodeposition, sol-gel process, and anodic oxidation. Among them, anodic oxidation proves to be a particularly effective method to generate self-organized nanotube arrays, whose tailorable porous morphology and high surface area are especially attractive for the applications as sensors, catalysts and dye-sensitized solar cell materials. Besides Ti, anodic oxidation has been applied to other metals (Al, Ta, Zr, Nb, Hf, and Fe) to conveniently synthesize metal oxide-based porous nanotube arrays.
Hitherto porous TiO2 photonic films are generally fabricated by lithographic methods or templating methods. Lithographic methods provide precision but are generally complicated and labor-intensive. The templating method is more convenient, which involves filling the voids of the self-assembled nanobead array with TiO2 and subsequent removal of the nanobead array, but it is more prone to structural defects. Comparing to the porous TiO2 photonic films, non-porous TiO2 photonic films have been more conveniently fabricated by electroplating or thermal oxidization, where a dense layer of TiO2 is either electroplated on a metal surface or generated on the surface of a titanium substrate by thermally oxidizing. The photonic response of these fabricated non-porous TiO2 films originates from the interference between the reflections of light from the surfaces of the film, or Fabry-Pérot interference. Thus, the color of these Fabry-Pérot TiO2 films can be adjusted by tuning the film thickness, which is relatively easy for the electroplating method but usually difficult for the thermal oxidization method. In general, dense TiO2 photonic films are not suitable for the applications as sensors, catalysts and dye-sensitized solar cell materials due to its non-porous feature and low surface area.